1. Field of the Invention
The present invention relates to infrared radiation detectors and, more particularly, to small bandgap semiconductor infrared detectors and methods of fabrication.
2. Description of the Related Art
Detection of infrared radiation emitted by warm bodies provides an important method for night vision (perception without visible light). Infrared detectors are of various types and include small bandgap semiconductors structured as photodiodes or photocapacitors. Alloys of mercury telluride and cadmium telluride, generically denoted Hg.sub.1-x Cd.sub.x Te, are extensively employed as the photosensitive semiconductor in such detectors. Indeed, Hg.sub.0.8 Cd.sub.0.2 Te has a bandgap of about 0.1 eV, and a 0.1 eV photon has a wavelength of 12 .mu.m; whereas, Hg.sub.0.73 Cd.sub.0.27 Te has a bandgap of about 0.24 eV, and a 0.24 eV photon has a wavelength of 5 .mu.m. These two wavelengths are in the two atmospheric windows of greatest interest for infrared detectors.
An infrared imager incorporating an array of MIS photocapacitor detectors in Hg.sub.1-x Cd.sub.x Te is disclosed in U.S. Pat. No. 4,684,812 (Tew and Lewis), hereby incorporated by reference. FIGS. 1a-b are cross sectional elevation and plan views of a single photocapacitor and illustrate the transparent nickel gate. There is a tradeoff between transmittance and electrical conductivity of the nickel gate, and typically the transmittance of 10 .mu.m wavelength photons is 50-60% with a sheet resistance of 100-200 .OMEGA./.quadrature.. Transmittance as high as 75% has been demonstrated but the nickel film becomes so thin (about 50 .ANG.) that discontinuities begin to appear and manufacture becomes difficult. Further, manufacture of Hg.sub.1-x Cd.sub.x Te containing devices requires processing temperatures below 200.degree. C. to avoid decomposition of the Hg.sub.1-x Cd.sub.x Te, and this aggravates discontinuity problems. But the trend for infrared imagers is to a transmittance of at least 70%; and even thinner nickel gates will be called for.
Thus it is a problem of the known infrared transparent gates to simultaneously achieve high transmittance, high electrical conductivity, and manufacturability.